Mixed analog and digital devices utilize analog-to-digital converters (ADCs) to convert the voltages of analog signals to corresponding digital values for use by digital components of the devices. Redundant signed digit (RSD)-based ADCs often find particular benefit in certain types of systems, particularly where power and space are at a premium. RSD ADCs typically convert an analog signal to a corresponding digital value through a series of stages. During the initial state, the voltage of the input analog signal is compared to two or more reference voltages, e.g., VH and VL, and the results of these comparisons result in code bits for the initial stage. An analog circuit comprising an amplifier and a set of capacitors is used to determine a residue voltage, and for the second stage the process of comparisons with the reference voltages is repeated with the residue voltage to generate code bits for the second stage. This process of calculating the residue voltage from the residue voltage of the previous stage and comparing the resulting residue voltage to generate code values can be repeated for a number of stages until the appropriate resolution is reached. An RSD algorithm then is applied to the code values from each stage to generate a digital value representative of the analog signal.
In some operating environments, different analog signal sources may utilize the same RSD ADC, but may operate at different voltage levels. To illustrate, in an automotive environment, different sensors may provide sensor output signals with different voltage levels for conversion to digital values for processing by the same control processor. In order to ensure proper conversion, each of input analog signals typically needs to be scaled to a predetermined voltage level before conversion. In conventional devices, this scaling is achieved through gain circuitry prior to the input of the RSD ADC. This separate gain circuitry complicates the design and integration of an RSD ADC, as well as adding to the size and power consumption of the integrated circuit in which the RSD ADC is implemented. Accordingly, an improved technique for scaling analog signals for digital conversion would be advantageous.